The present invention relates to an optical recording medium of high density represented by an optical disc. In particular, the present invention relates to an optical recording medium having a surface in which the number of recesses and projections is small, and having a reflective layer excellent in characteristics. Further, the present invention relates to an optical recording medium capable of improving the surface properties of the reflective layer without damaging properties such as high thermal conductivity, high reflectivity, excellent productivity and so on, and providing excellent recording/retrieving characteristics.
In many optical discs, a reflective layer containing a metal as the main component is used. In an optical disc such as an optical disc exclusively used for retrieving, CD-R or the like, the reflective layer functions to increase the quantity of returning light by reflecting light. Although the reflectivity of the medium such as a magneto-optical recording medium, a phase-change recording medium or the like, is not so high, light reflected at the reflective layer is utilized to increase the intensity of signals. Further, the reflective layer having a high thermal conductivity is utilized for solving a problem of heat. Namely, it is preferable that the reflective layer has sufficient reflectivity and thermal conductivity. In particular, the high thermal conductivity is important for a rewritable phase-change optical disc wherein the cooling speed after the temperature rise by irradiation of light to the medium influences largely the formation of recording marks.
In recent years, there has been proposed a system to obtain a further high density wherein the numerical aperture (NA) of an objective lens in an optical system used for recording or retrieving is made large, and the lens is brought closer to the medium. In this case, in order to bring the objective lens close to the recording layer as possible, it is preferable to conduct recording or retrieving from a layer surface side (the surface side of the recording layer on the side opposite to the substrate) without passing light through the substrate having a larger thickness. There is another problem that since an objective lens having a high NA has a larger thickness, it is difficult to bring the lens close to the medium. Further, there has been made an attempt to introduce light from the layer surface side but not from the substrate side by the reasons of reducing the deterioration of the characteristics due to an inclination of the substrate, to reduce the birefringence of the substrate and so on.
When a metallic layer having sufficiently high reflectivity and thermal conductivity is formed by sputtering or the like, the surface roughness of the layer tends to increase as the layer thickness becomes thick. Accordingly, the surface roughness of the metallic layer (the surface of the metallic layer on the side opposite to the substrate) becomes inevitably large, whereby the surface roughness of the reflective layer is generally large.
FIG. 3 is an example of the layer structure of a substrate side incident type optical disc. A protective layer 2, a recording layer 3, a protective layer 4 and a reflective layer 5 are laminated on the substrate 1 wherein the reflective layer 5 has a large surface roughness. However, the reflective layer is generally formed to have a certain thicker dimension so that the light transmittance of light for recording or retrieving becomes small. Accordingly, when light 10 is introduced from a substrate side, the almost amount of light is reflected at a rear surface of the reflective layer, and accordingly, the surface roughness of the reflective layer does not substantially influence the recording or retrieving.
On the other hand, circumstances differ in a case of introducing light from a layer surface side. FIG. 2 is an example of the layer structure of a layer surface side incident type optical disc. A reflective layer 5, a protective layer 4, a recording layer 3 and a protective layer 2 are laminated on the substrate 1. Light 10 introduced from a layer surface side is reflected at a front surface of the reflective layer having a large surface roughness as shown in FIG. 2. Accordingly, there is a problem that the surface roughness of the reflective layer influences largely the recording/retrieving characteristics. Specifically, there is a problem that noises in recording or retrieving become large.
Further, there is considered the same problem in a case of forming the recording layer after a thin translucent metallic layer has been formed (in a case of forming a translucent metallic layer between the substrate 1 and the recording layer 3 in FIG. 3) even when light is introduced from a substrate side.
These problems become remarkable as the size of laser beams is smaller. Therefore, when the laser wavelength is made short or the numerical aperture NA of the objective lens is made large in order to perform high density recording, a large problem will arise.
The phenomenon that a metallic layer having high reflectivity and thermal conductivity has a large surface roughness is related largely to crystal grains. For example, a low growing speed of crystallization at the grain boundary of crystal grains is one of the factors.
In view of the above, there have been proposed techniques to improve the surface properties of the layer: e.g., a method for making the crystal grain size fine and uniform by mixing impurities in the reflective layer, a method for conducting reverse sputtering after the formation of the reflective layer (JP-A-2000-228033), a method for making the crystal grains in the reflective layer fine and uniform by using particle of Cr or the like as crystal nuclei of the material for the reflective layer, and so on.
However, any of the above-mentioned methods has a drawbacks. Namely, the method for mixing impurities in the material of the reflective layer of metal reduces its thermal conductivity or reflectivity by mixing impurities. The method for conducting reverse sputtering after the formation of the reflective layer is not suitable for mass production because a layer forming process is complicated and takes much time. Further, the method for using particles of Cr or the like as crystal nuclei has difficulty in controlling the particles of Cr or the like to have a size suitable for the crystal nuclei for the reflective layer and is not suitable for productivity. In short, these methods could not provide the reflective layer excellent for all requirements such as thermal conductivity, reflectivity, surface properties, good productivity and so on.
It is an object of the present invention to eliminate the above-mentioned problems and to provide an optical recording medium capable of improving the surface properties of the reflective layer without damaging the properties such as thermal conductivity, reflectivity, good productivity and having excellent recording/retrieving characteristics.
The inventor of this application has achieved the present invention by finding that the crystal grain size can be made fine and uniform, without a special contrivance to the reflective layer itself, by forming a specified layer containing the same metallic element as the metal constituting the main component of the reflective layer on the surface of the reflective layer facing the substrate, whereby the reduction of noises in retrieving can be achieved.
In accordance with a first aspect of the present invention, there is provided an optical recording medium for recording and/or retrieving information by irradiation of light, which comprises a substrate, and an interlayer, a reflective layer containing a metal as the main component and a recording layer, formed in this order on the substrate directly or via another layer made of a resin or a dielectric material, wherein as observed from a light-incoming direction, the reflective layer is located in front of the interlayer, the interlayer contains the same metal as the metal constituting the main component of the reflective layer and also contains oxygen and/or nitrogen, and the interlayer has a content of the metal smaller than the reflective layer. The main component means 50 atomic % or more.
In accordance with a second aspect of the present invention, there is provided an optical recording medium for recording and/or retrieving information by irradiation of light, which comprises a substrate, and a crystal grain size-controlling layer, a reflective layer containing a metal as the main component and a recording layer, formed in this order on the substrate directly or via another layer made of a resin or a dielectric material, wherein as observed from a light-incoming direction, the reflective layer is located in front of the interlayer, and the crystal grain size-controlling layer has a function to make the crystal grain size of the metal forming the reflective layer fine and uniform.
In accordance with a third aspect of the present invention, there is provided a process for producing an optical recording medium for recording and/or retrieving information by irradiation of light, which comprises a substrate, and an interlayer, a reflective layer containing a metal as the main component and a recording layer, formed in this order on the substrate directly or via another layer made of a resin or a dielectric material, wherein as observed from a light-incoming direction, the reflective layer is located in front of the interlayer, and the interlayer contains the same metal as the metal constituting the main component of the reflective layer and also contains oxygen and/or nitrogen, said process comprising sputtering the metal onto the surface of said substrate or said another layer while introducing oxygen and/or nitrogen into the atmosphere, to form said interlayer, and then, sputtering the metal without introducing oxygen and/or nitrogen, to form said reflective layer.
As one of the features of the present invention, the crystal grain size-controlling layer having a function to make the crystal grain size of the metal forming the reflective layer thin and uniform, is formed previously under the reflective layer of metal. The inventors of this application have found that with such structure, the surface properties of the reflective layer can be improved, and noises from the optical recording medium can be reduced. Further, there is no possibility of damaging the thermal conductivity and the reflectivity of the reflective layer. Further, since the crystal grain size-controlling layer can be formed easily, excellent productivity is obtainable.
Further, the inventors have found that the interlayer having a specified composition is excellent as the crystal grain size-controlling layer for the reflective layer of metal. In addition, they have found the process for producing the optical recording medium with good productivity.